Effect of structural change in viologen acceptors on the rate of single electron transfer from tributylphosphine

The "flexible" 3 and "rigid" cyclic viologens 4, diquarternary salts of 2,2'-bipyridine and 1,10-phenanthroline, respectively, were treated with tributylphosphine (1) in acetonitrile containing a large amount of methanol under an argon atmosphere. A single electron transfer (SET) easily occurred from the latter to the former, the SET to 4 being 10(5)-10(6) times faster than the SET to 3. The reorganization energy lambda for the latter SET is thought to be larger than that for the former SET, because 3 undergoes a structural change upon the one-electron reduction to its radical cation, whereas the one-electron reduction of 4 takes place without a structural change. Taking into account the difference in lambda, and also taking into account the bond formation energy brought about by the follow-up reaction of the phosphine radical cation 1*(+) with methanol, the observed kinetics were well interpreted in terms of the Marcus theory.     

Photoinduced electron transfer between metal-coordinated cyclodextrin assemblies and viologens

Two novel tris(bipyridine)ruthenium(II) complexes bearing two and six beta-cyclodextrin binding sites on their ligands have been synthesised and characterised. Complex 1, bearing two cyclodextrins, adopts a conformation in aqueous solution where parts of the aromatic ligands are self-included into the cyclodextrin moieties. This results in a loss of symmetry of the complex and gives rise to a much more complicated 1H NMR spectrum than expected. Photophysical studies indicate that the appended cyclodextrins protect the luminescent ruthenium core from quenching by oxygen, which results in longer excited state lifetimes and higher emission quantum yields compared with the reference compound, the unsubstituted ruthenium tris(bipyridine). Inclusion of suitable guests such as dialkyl-viologens leads to a quenching of the luminescence of the central unit. In these supramolecular donor-acceptor dyads an efficient photoinduced electron transfer from the excited ruthenium moiety (the donor) to the viologen unit (the acceptor) is observed. The alkyl chain length of the acceptor plays an important role on the binding properties; when it exceeds a certain limit the binding becomes strong enough for electron transfer to occur. Interestingly, a viologen with only one long alkyl tail instead of two shows no efficient quenching; this indicates that cooperative interactions between two cyclodextrins binding one viologen are essential to raise the binding constant of the supramolecular dyad.     

Luminescence quenching by DNA-bound viologens: effect of reactant identity on efficiency and dynamics of electron transfer in DNA

Photoinduced electron transfer from two intercalating photoactive donors, Ru(phen)2dppz2+ and ethidium, to intercalating viologen acceptors of the N,N'-dialkyl-6-(2'-pyridiniumyl)phenanthridinium family has been investigated through steady-state and time-resolved luminescence quenching measurements. Efficient quenching of the emission from these donors bound to DNA is observed at low concentrations of acceptor (1-10 eq.), and in time-resolved emission experiments it is determined that electron transfer occurs on the nanosecond time scale. Furthermore, transient absorption measurements confirm that the quenching is the result of a charge-transfer process; upon photoreaction of intercalated Ru(phen)2dppz2+ with a viologen acceptor, an intermediate with spectral properties resembling the expected charge-separated pair is observed. The quenching yields and kinetics obtained with this quencher are in marked contrast to those observed with these same donors paired with Rh(phi)2bpy3+ as an acceptor. The differing efficiencies of electron transfer for these donor/acceptor pairs bound to DNA as compared to others previously described are discussed qualitatively in terms of the structural and electronic properties of the different reactants.     

Dramatic steric behavior in electron transfer from various donors to CF3Br

Different alkali metal atoms are observed to donate electrons to CF(3)Br molecules that are oriented in space. For collision energies high enough to overcome the Coulomb attraction, a positive ion/negative ion pair is observed and mass-analyzed using coincident time-of-flight mass spectroscopy. The alkali metal cation and various negative ions are observed. The most abundant negative ion is the bromide ion, Br(-), formed preferentially by attack at the Br end of the molecule. The steric asymmetry to produce Br(-) is almost identical for all of the alkali metal donors. Fluoride ions are formed in smaller abundance and reflect completely different behavior among the donors. Sodium and potassium have high thresholds and prefer the CF(3) end of the molecule, whereas cesium prefers the Br end of the molecule. Sodium and potassium apparently interact with the transient CF(3)Br(-) molecular negative ion while it is in the process of decomposing.     

Reduction of aldehydes and ketones by sodium dithionite using viologens as electron transfer catalyst

It was found that aromatic aldehydes and ketones could be reduced smoothly at room temperature by sodium dithionite using viologen derivatives as an electron transfer catalyst to obtain the corresponding alcohols in good yields. Viologens (methyl viologen and polymer) acted catalytically and active species in the reduction were quinoid forms which were obtained by two-electron reduction of viologens.     

Immobilization of heptyl viologens in DNA strands both to inhibit dimerization and to accelerate quasi-reversible electron transfer reaction

Viologens, mono-molecularly immobilized in DNA grooves, show reversible colour changes without attenuation during potential switching after covering with ionic liquid type polymers.     

Intermittent electron transfer activity from single CdSe/ZnS quantum dots

Electron transfer activity from excited single CdSe/ZnS core/shell quantum dots (QDs) to adsorbed Fluorescein 27 was studied by single QD fluorescence spectroscopy. In comparison with QDs, the QD-acceptor complexes showed a shorter average and broader distribution of QD emission lifetimes due to electron transfer to adsorbates. Large fluctuation of lifetimes in single QD/dye complexes was observed, indicating intermittent electron transfer activity from QDs.     

Photocatalytic single electron transfer reactions on TiO2 semiconductor

The use of inorganic semiconductor particles such as titanium dioxide(TiO_2) has received relatively less attention in organic chemistry, although semiconductor particles have been widely used as a single electron transfer photocatalyst in waterpurification, air-cleaning, and self-cleaning. In recent years, the photocatalysis on semiconductor particles has become an active area of research even in organic chemistry, since the heterogeneous semiconductor photocatalysis leads to the unique redox organic reactions. In an early stage, the semiconductor photocatalysis was applied to the oxidation of organic molecules.Semiconductor particles have also the potential to induce the reductive chemical transformations in the absence of oxygen(O_2),by using the suitable sacrificial hole scavenger. In this review, we summarize the representative examples of the reductive and oxidative organic reactions using semiconductor particles and the recent applications to the stereoselective reactions.     

Solvent friction effect on intramolecular electron transfer

U-shaped donor-bridge-acceptor molecules with different electronic couplings have been investigated as a function of temperature in solvents with slow polarization relaxation, in particular, N-methylacetamide (NMA) and N-methylpropionamide (NMP). At high temperature, the electron-transfer rate is well described by a nonadiabatic model; however, the rate at low temperature is controlled by the solvent friction. The change of the electron-transfer mechanism is discussed and compared with theoretical models.     

Unimolecular and bimolecular transfer of N-substituents from pyridinium cations: Evidence for a clear mechanistic changeover

$\text{N}$-Substituents in 2, 4, 6-triphenylpyridiniums are transferred to piperidine, morpholine and pyridine by unimolecular and/or bimolecular processes in chlorobenzene solution. These processes are quite distinct and afford no evidence for a mechanism intermediate between S_N1 and S_N2.     

First single electron transfer reaction on propargylic chloride in 5-nitroimidazole series

We report here the first example of an S_RN1 reaction on propargylic chloride in heterocyclic series. The reaction of 4-(3-chloroprop-1-ynyl)-1,2-dimethyl-5-nitro-1H-imidazole with nitronate anions led to both the formation of the C-alkylated product through an S_RN1 mechanism and the predominant ethylenic compound resulting from nitrous acid elimination on the C-alkylated product. Interestingly, in contrast to our previous works on S_RN1 reactivity, no O-alkylated product was observed.     

Excited state interfacial electron transfer from a compound with a single pyridine ligand

The coordination compound Ru(NH(3))(5)(eina)(PF(6))(2), where eina is ethyl isonicotinate, was synthesized and attached to optically transparent nanocrystalline (anatase) TiO(2) films, abbreviated Ru(NH(3))(5)(eina)/TiO(2). The metal-to-ligand-charge-transfer (MLCT) absorption was found to shift in wavelength with solvent. The absorption maximum of the low energy MLCT band was observed at 486 nm in acetonitrile and 528 nm in dimethylformamide for Ru(NH(3))(5)(eina)(PF(6))(2) and at 512 and 555 nm for Ru(NH(3))(5)(eina)/TiO(2), respectively. The compound was found to be nonemissive with an excited state lifetime <10 ns under all conditions studied. Light excitation in fluid solution and when attached to insulating ZrO(2) films resulted in a loss of the MLCT absorption, consistent with ligand field photochemistry. Pulsed light excitation of Ru(NH(3))(5)(eina)/TiO(2) yields an absorption difference spectrum consistent with an interfacial charge separated state, Ru(III)(NH(3))(5)(eina)/TiO(2)(e(-)). This state forms within 10 ns and returns cleanly to ground state product within milliseconds. The injection quantum yields were determined by comparative actinometry and were found to be excitation wavelength dependent: phi(inj)(417 nm) = 0.30 +/- 0.05 and phi(inj)(532.5 nm) = 0.15 +/- 0.03. Regenerative solar cells based on Ru(NH(3))(5)(eina)/TiO(2) with 0.5 M TBAI, where TBA is tetrabutylammonium, and 0.05 M I(2) in acetonitrile were very inefficient. Sluggish iodide oxidation is expected, on the basis of the negative E degrees (Ru(III/II)) = +0.17 (V vs Ag/AgCl) reduction potential, and this presumably allows a greater fraction of the injected electrons to recombine with the oxidized compound thereby lowering the solar cell efficiency.     

Studying and switching electron transfer: from the ensemble to the single molecule

We report here on the systematic investigation of photoinduced intramolecular electron transfer (IET) in a series of donor-bridge-acceptor molecules as a means of understanding electron transport through the bridge. Perylenebisimide chromophores connected by various oligophenylene bridges are studied because their electron-transfer behavior can readily be monitored by following changes in the fluorescence intensity. We find dramatic switching of the IET behavior as the solvent polarity (dielectric constant) is increased. By combining steady-state and time-resolved fluorescence spectroscopy in a variety of solvents at multiple temperatures with standard theories of electron transfer, we determine parameters governing the IET behavior of these dimers, such as the electronic coupling through the bridges. We also deploy available ab initio quantum chemical methods to calculate the through-space component of the electronic coupling matrix element. Single-molecule investigations of the electron-transfer behavior also show that IET can be switched reversibly by a similar mechanism in an isolated individual molecule.     

Influence of polystyrenesulfonate on electron transfer quenching of ruthenium trisbipyridine luminescence by viologens: non-covalent assembly and covalent tethering of the ruthenium complex

A new copolymer (RuB-PSS) of ruthenium(II)bis-(2,2'-bipyridine)(4-vinyl 2,2'-bipyridine) and styrene sulfonate was prepared which tethers the ruthenium chromophore directly to the polymer backbone. The photophysical properties of the copolymer, and its luminescence quenching by viologens, were compared with those of ruthenium(II)tris-bipyridine, [Ru(bpy)(3)](2+), bound non-covalently to polystyrenesulfonate (PSS) via hydrophobic and electrostatic interactions. Enhancement of ruthenium polypyridyl complex luminescence in both systems is due to decreased rates of non-radiative decay when removed from bulk water as well as reduced oxygen quenching. Molecular dynamics simulations show an open PSS chain conformation with induction of local curvature around the ruthenium centres. Hence, the complexes remain exposed to water, albeit less so than in bulk solution, as evidenced by low enhancement of bound [Ru(phen)(2)dppz](2+) emission. Quenching by O(2) is hindered for both systems due to combined polarity, ionic strength, and viscosimetric effects that influence local concentrations and diffusion of reactants. Electron transfer quenching of the Ru centre by zwitterionic propyl viologen sulfonate (PVS(0)) and cationic methyl viologen (MV(2+)) is enhanced for [Ru(bpy)(3)](2+)/PSS, but retarded for RuB-PSS, despite the attraction of the quenchers for PSS. PSS binding hinders separation of the electron transfer products relative to aqueous solution, excepting an increase for RuB-PSS/PVS(0). We conclude that anionic hydrophobic polymers such as PSS can differentially influence forward- and reverse- electron transfer reactions depending on the charge and hydrophobicity of the reactants. In the context of small molecule binding, we find that PSS provides a tenable model for DNA.     

光敏电子转移异构反应中的盐效应

本文利用四正丁基四氟硼酸铵为探针, 研究了顺式芪和四环烷的9,10-二氰基蒽敏化光异构化的反应机理, 加入四正丁基四氟硼酸铵明显加快顺式芪的异构反应而减慢四环烷的反应。荧光猝灭及激光闪光光解实验证明四正丁基四氟硼酸铵能促进电荷分离过程而生成离子自由基对。从而证实顺式芪的异构化反应是经由离子自由基的历程, 而四环烷则是通过激基复合物机理。     

N-heterocyclic superelectrophiles and evidence for single electron transfer chemistry

In reactions with benzene and related substrates, an oxazole-based superelectrophile is found to be significantly more reactive than a related monocationic species. Theoretical calculations estimate that the lowest unoccupied molecular orbital (LUMO) for the superelectrophile is about 4 eV lower in energy than the LUMOs of comparable monocations. When the oxazole-based superelectrophile is reacted with ferrocene, a dimeric product is formed in high yield. The dimerization occurs by a single electron transfer reaction between the dicationic superelectrophile and ferrocene, followed by coupling of the radical cations.     

Unravelling single metalloprotein electron transfer by scanning probe techniques

This review is intended to account for the experimental and theoretical achievements obtained in a period of about 15 years on the investigation of the electron transport through single redox metalloproteins by scanning probe techniques. A highly focussed research effort has been deployed by the scientists active in this particular field towards measuring and interpreting electronic current signals flowing via blue copper, redox metalloproteins (e.g. azurin). The field has taken a remarkable advantage of the use of electrochemically assisted scanning tunnelling microscope (EC-STM) which has allowed to probe single molecule signals under full control of all the potential values involved in the experiments. This experimental activity has both triggered more comprehensive theoretical interpretations and has been, in its turn, stimulated by theoreticians to test always new predictions. The authors hope to have succeeded in providing the reader with a valuable appraisal of this fascinating field.     

Photoinduced electron transfer in naphthalimide-pyridine systems: effect of proton transfer on charge recombination efficiencies

We studied the effect of proton-coupled electron transfer on lifetimes of the charge-separated radicals produced upon light irradiation of the thiomethyl-naphthalimide donor SMe-NI-H in the presence of nitro-cyano-pyridine acceptor (NO(2)-CN-PYR). The dynamics of electron and proton transfer were studied using femtosecond pump-probe spectroscopy in the UV/vis range. We find that the photoinduced electron transfer between excited SMe-NI-H and NO(2)-CN-PYR occurs with a rate of 1.1 × 10(9) s(-1) to produce radical ions SMe-NI-H(?+) and NO(2)-CN-PYR(?-). These initially produced radical ions in a solvent cage do not undergo a proton transfer, possibly due to unfavorable geometry between N-H proton of the naphthalimide and aromatic N-atom of the pyridine. Some of the radical ions in the solvent cage recombine with a rate of 2.3 × 10(10) s(-1), while some escape the solvent cage and recombine at a lower rate (k = 4.27 × 10(8) s(-1)). The radical ions that escape the solvent cage undergo proton transfer to produce neutral radicals SMe-NI(?) and NO(2)-CN-PYR-H(?). Because neutral radicals are not attracted to each other by electrostatic interactions, their recombination is slower that the recombination of the radical ions formed in model compounds that can undergo only electron transfer (SMe-NI-Me and NO(2)-CN-PYR, k = 1.2 × 10(9) s(-1)). The results of our study demonstrate that proton-coupled electron transfer can be used as an efficient method to achieve long-lived charge separation in light-driven processes.     

Notable deuterium effect on the electron transfer rate of myoglobin

The electron transfer reaction of wild-type myoglobin at an electrode was significantly facilitated in a D2O buffer as compared with that in an H2O buffer, with k(0)'(H2O)/k(0)'(D2O)= 0.13, while a minimal deuterium kinetic isotope effect on the myoglobin with modification at distal histidine (His-64) was observed.